Catheters are tube-like medical instruments that are inserted into a body cavity organ or blood vessel for diagnostic or therapeutic reasons. Medical vascular catheters are particularly designed for insertion into the vasculature and are available for a wide variety of purposes, including diagnosis, interventional therapy, drug delivery, drainage, perfusion, and the like. Medical vascular catheters for each of these purposes can be introduced to numerous target sites within a patient's body by guiding the catheter through an incision made in the patient's skin and a blood vessel and then through the vascular system to the target site.
Medical vascular catheters generally comprise an elongated, flexible catheter tube or body with a catheter side wall enclosing a catheter lumen extending between a catheter body proximal end coupled to a relatively more rigid catheter hub to a catheter body distal end. The catheter body may be relatively straight or inherently curve or curved by insertion of a curved stiffening wire or guide wire through the catheter lumen. The catheter body and catheter side wall are typically fabricated and dimensioned to minimize the catheter body outer diameter and side wall thickness and to maximize the catheter lumen diameter while retaining sufficient side wall flexibility and strength characteristics to enable the catheter to be used for the intended medical purpose.
One of the therapeutic procedures applicable to the present invention is known as percutaneous transluminal coronary angioplasty ("PTCA"). PTCA can be used, for example, to reduce arterial build-up of cholesterol fats or atherosclerotic plaque. Catheters must have sufficient stiffness to be pushed through vessels as well as rigidity to provide a high degree of torsional control. Stiffness or rigidity in the catheter tip poses the danger of puncturing or otherwise damaging a vessel as it twists through the vascular system. It is therefore desirable for catheters to have a soft or flexible distal tip. The trend toward thin side wall catheters of less than 0.3 mm wall thickness and a softer distal tip results, however, in a substantially weaker bond between the distal soft tip and the catheter shaft.
Commonly-owned U.S. Pat. Nos. 5,509,910 issued to Lunn and 5,545,149 issued to Brin et al. describe various prior art methods of attaching distal soft tips to proximal catheter shafts and their improvements upon those methods. In the '910 and '149 patents, a composite proximal catheter shaft is employed that is formed of an outer tube or sheath, an inner liner surrounding the catheter lumen and a wire braid reinforcing layer between the outer sheath and inner liner. In both cases, a distal soft tip is attached at the distal end of the catheter shaft through the employment of an intermediate segment. The presence of wire braid and TEFLON.RTM. (polytetrafluoroethylene or PTFE) in a typical multi-layer catheter shaft compromises the bond between the catheter shaft and a distal tip segment since the materials used for the soft tip do not bond well to wire braid or to TEFLON.RTM.. Thus, the transition segment is utilized between the distal end of the catheter shaft and the distal tip segment which is comprised of materials with a high tensile strength relative to the materials comprising the soft tip. As a result, the high strength of the transition segment compensates for the compromised bonding with the multi-layer catheter shaft and yields acceptable bond strength with the distal soft tip. The use of a high tensile strength transition segment is particularly important to achieving acceptable bond strength where the catheter wall thickness is less than 0.3 mm and when the soft tip material is a low tensile strength material, such as Shore 80A Pellethane.RTM. polyurethane.
In the '910 patent, a high tensile strength transition segment, selected from a group of thermoplastic elastomers having an ultimate tensile strength of at least 45 MPa is injection molded between the pre-formed distal soft tip and the distal end of the catheter shaft. The transition segment is injected in a molten state to encapsulate the modified surface geometries of the distal catheter shaft and the proximal soft tip segment to create the improved lap joint. Furthermore, the surface geometry of both the distal end of the elongated tubular shaft and the distal tip segment are modified to reduce stress concentration and increase surface area so that the adequate bonding to the distal tip segment is achieved. The use of a high tensile strength transition segment coupled with a modified surface geometry substantially increases the tensile strength of the bond between the catheter shaft and the distal soft tip. In the preferred embodiment, a modified surface geometry on the distal end of the catheter shaft is created by removing two ungular sections on either side of the central longitudinal axis of the catheter shaft. Similarly, the surface geometry on the proximal end of the tip segment is modified by removing two ungular sections on either side of the central longitudinal axis of the tip segment. An alternative embodiment is the removal of more than two ungular sections on either or both of the distal end of the catheter shaft or the proximal end of the soft tip segment to further promote bonding.
In the '149 patent, an improved method of soft tip attachment is disclosed where lap joints are formed between a pre-formed distal soft tip and a pre-formed intermediate transition tube or segment and between the transition segment and the distal end of a reinforced proximal catheter shaft. In this case, the pre-formed parts are assembled on a mandrel and within a heat shrink tube, and the assembly is subjected to IR heating. The applied heat and the pressure of the shrinkage of the heat shrink tube causes localized melting to take place at the abutting ends of the transition segment with the distal end of the catheter shaft and the proximal end of the distal soft tip. The heat shrink tube and the mandrel are removed after the assembly is cooled and the joints have solidified.
Radio frequency (RF) energy sources have also been employed to thermally bond a pre-formed distal soft tip to a wire braid reinforced catheter shaft. The RF energy is applied to the parts assembled over a mandrel, and the heat causes the materials to fuse at the butt joint.
A problem which arises with these approaches is that the lap joints or butt joints which bond the catheter shaft, transition segment, and distal soft tip are attained through substantial pressure and/or heat which can have the adverse effects on the concentricity, stiffness, and kink resistance of the catheter shaft. In general, it is desirable to reduce the thickness of the catheter shaft side wall and the distal soft tip and any intermediate segments as much as possible to maximize the catheter lumen ID for any specified French size catheter. The thickness reduction results in less bonding surface area for the lap or butt joints and also causes the wire braid to become exposed through the thin outer sheath as the sheath material contracts into the interstitial spaces of the braid when heat is applied to form the bond. The exposed braid makes the catheter body rough or irregular at the junction and the thinning can weaken the side wall.
Thus, an improved soft tip is needed which provides adequate bond strength to the catheter shaft where the wall thickness of the catheter shaft is less than 0.3 mm and the tip material is of the requisite softness without compromising concentricity, stiffness, or kink resistance of the catheter shaft. The present invention solves this problem.